Oh for fuck's sake, these characters are so dumb.
The son has such an inconsistent series of motivations. It makes sense he wants to avenge his father, but he flips on a dime for a 2-year long revenge scheme because of the word a girl he knows is close to MC, so we didn't he assume she was lying to cover for his ass and this was some long-winded scheme of his to get back at him? Wouldn't it be more tragic and consistent if she told him the truth and yet he refused to believe it because of the scheme? Yet he somehow is convinced from one conversation his entire world view shifts.
Also, the argument he makes to demonize MC is full of fallacies, not only does use ad hoc logic to assume the PRETEEN MC would be at fault for the stress everyone ELSE did TO his family, which were not his fault. Also, given he's trying to date his sister whilst harassing his mother and operating a SNS that impersonates him, he's speaking completely as a hypocrite. Normally this wouldn't be an issue if the characters called him out, and the narrative painted this criticism as the desperate, insane logic of someone too emotionally driven but that he is correct.
Let's not forgot MC is portrayed as basically flawless, because he's just too selfless that it harms his social standing with his family.
Also, he forgets that even if he's 17, he can be charged as an adult, especially if it was premediated.
On a side note, how the fuck did no one notice the man holding a knife against a girl's throat in a busy train yard?